In fast-neutron nuclear reactors cooled by a liquid metal, the large-size main vessel contains liquid metal, in which the reactor core is immersed. This vessel generally comprises a cylindrical side wall and a dished bottom, and is suspended on the structure of the reactor by means of the upper end of its cylindrical wall.
The level of liquid metal in the main vessel is such that the upper part of the cylindrical wall is in contact with the sodium up to a certain height, and as a result of conduction the submerged part is therefore at the same temperature as the sodium in contact with it. The upper part of the main vessel in contact with gas has a temperature which decreases virtually linearly up to its fixing point.
In order to perform its function of supporting the core and as an intermediate safety containment, the vessel must be kept cold during its normal operation (the version with negligible creep). Moreover, the system must be such that the free level of sodium in contact with the vessel is fixed, in order to avoid problems of progressive deformation linked to the variations in level.
Consequently, this upper part of the main vessel located in line with the hot collector must be kept cold and with a fixed level of sodium.
Of the many solutions, the most common involves circulating cooled sodium in contact with the inner surface of the wall of the vessel in its upper part. This cooled or cold sodium is taken up at the outlet of the primary pumps and comes from the cold collector of the reactor, into which opens the outlet of the intermediate exchangers. Suitable means ensure that this cold sodium is injected into a first collector or overflow collector of annular shape, formed between the upper part of the wall of the main vessel of substantially cylindrical shape, and a sleeve, called an overflow sleeve, coaxial relative to the wall of the main vessel and of a diameter less than the diameter of this vessel. The height of the sleeve and that of the overflow collector correspond to a fraction of the height of the vessel.
The cold sodium received in the overflow collector flows into a second annular collector, called a return collector, over the top of the upper end of the overflow sleeve. The return collector is delimited by the overflow sleeve forming its outer wall and by an inner sleeve of substantially cylindrical shape, coaxial relative to the overflow sleeve and to the wall of the vessel and of a diameter less than that of the overflow sleeve. Holes made in the bottom of the return collector allow the liquid sodium to return into the cold collector of the main vessel. These holes are provided as a function of the inflow rate of the cold sodium into the return collector, so that the sodium level in this collector is set at a certain height below the sodium overflow level corresponding to the upper end of the overflow sleeve.
The overflow collector is usually supplied with cold sodium via pipes putting it into communication with the flooring supporting the bearer receiving the feet of the assemblies of the core, into which the cold sodium is injected by means of the pumps.
In prior art devices, the overflow sleeve and the inner sleeve are connected to the same bottom, with the result that the two collectors have a substantially equivalent height. On the other hand, the effective head of the sodium corresponding to the difference in level in the two collectors is such that the overflow of sodium gives rise to phenomena of vibration of the vessel and of the sleeves delimiting the collectors.
In fact, the main vessels of fast-neutron nuclear reactors built or being designed at the present time have a large diameter of the order of twenty meters and a relatively small thickness of the order of twenty-five millimeters, making it possible to limit the thermal stresses and the weight of metal used for constructing the vessel. The collector sleeves themselves have dimensions near those of the vessel, and these units are extremely flexible and have natural periods of vibration which are very low and close to one another. Vibrations are therefore generated easily when the sodium overflows.